1. Field of the Invention
The present invention relates to communications, and, in particular, to the detection of frequency correction bursts during wireless communications between a base station and a mobile station.
2. Description of the Related Art
In wireless communications networks conforming to GSM/EDGE (Global System for Mobile Communications/Enhanced Data Rates for GSM Evolution) standards, base stations transmit frequency correction (FC) bursts and synchronization bursts. Mobile stations detect the FC bursts to derive frequency correction information that the mobile stations then use to decode the synchronization bursts that enable the mobile stations to synchronize their operations with those of the base stations.
According to the GSM/EDGE standards, an FC burst is ideally equivalent to an unmodulated carrier of 1625/24 kHz. In the real world, there will be carrier-frequency errors resulting from differences in the frequencies of the local oscillators at the base station and mobile station as well as possibly from Doppler effects due to motion of the mobile station relative to the base station. The purpose of FC bursts is to enable a mobile station to determine the carrier-frequency error so that subsequent processing, for example, of a synchronization burst can compensate for that carrier-frequency error. It is desirable to detect the end of an FC burst exactly so that all the samples of the FC burst can be used in carrier-frequency error estimation.
Prior-art techniques for detecting FC bursts include (1) cross correlation with a number of predetermined sequences and choosing the one with the maximum correlation; (2) application of an adaptive notch filter to the input signal, where adaptation is implemented using a least mean squares (LMS) algorithm, where an FC burst is detected based on the error power; and (3) predicting samples using an LMS algorithm and detecting an FC burst based on the error signal of the LMS algorithm. These prior-art techniques fail to provide accurate, robust detection for low signal-to-noise conditions.